Soil microbial activities and characteristics of dissolved organic C and N in relation to tree species

The aim of this study was to compare soil microbial biomass and activities with the characteristics of water-extractable organic matter in different types of forest stands. The study was carried out in adjacent 70-year-old stands dominated by silver birch (Betula pendula Roth.), Norway spruce (Picea abies (L.) Karst.) or Scots pine (Pinus sylvestris L.) growing in originally similar soil on Hylocomium-Myrtillus site type in northern Finland. The birch stand was a pure single-species stand, but the coniferous stands were mixed with birch. Soil samples were taken from the humus layer. Water extracts were analyzed for the concentrations of dissolved organic C and N (DOC and DON) and characterized according to chemical composition using a resin fractionation technique and molecular size distribution. C and N in the soil microbial biomass and rates of C mineralization, net N mineralization and net nitrification were measured. Soil pH varied from 4.0 to 4.3 and was highest in the birch stand. The C-to-N ratio of the soil organic matter was higher in the pine stand than in the other stands. Per unit organic matter, the amounts of C and N in the soil microbial biomass were highest in the birch stand. In laboratory incubation experiments, both the rate of C mineralization and that of net N mineralization were much higher in soils from the birch and spruce stands than in soil from the pine stand. This was also the order for concentrations of both DOC and DON. The DOC-to-DON ratio was similar in the birch and spruce soils but higher in the pine soil. The distributions of both different chemical and molecular size fractions were relatively similar in all soils, except for differences in some fractions of DON. The hydrophobic acid fraction was the major carrier of both DOC and DON, accounting for about 60% of DOC and 40-55% of DON. The DOC-to-DON ratio was especially low in the hydrophilic neutral fraction and in the smallest molecular size fraction (<1 kDa). Based on these results, the microbial biomass and activities appeared to be correlated with the total concentrations of DOC and DON rather than with their characteristics. (C) 2002 Elsevier Science Ltd. All rights reserved.